Cardiac glycosides (CG) have two well known effects on cardiac muscle, the inotropic effect and the inhibition of Na ion-K ion-ATPase. The major purpose of the present study is to correlate these two effects of CG by measuring changes of contractility and intracellular Na ion concentration simultaneously in isolated cardiac muscle following the administration of CG. Intracellular Na ion concentration will be monitored with specially designed Na ion-sensitive glass microelectrodes. Various experimental conditions which are known to influence these two effects will include many variations in ionic concentrations of the extracellular fluid, applications of antibodies to CG and testing of CG of different potencies. Since Ca ion movement$ through or interaction with cell components including cell membrane, sarcoplasmic reticulum, the modulatory proteins (troponin plus tropomyosin) and possibly mitochondria, during excitation-contraction cycle is critically important in cardiac contraction and Ca ions movement of interaction is known to be influenced by CG, the effect of CG on the interaction of Ca ion and these cell components will be investigated. For this purpose, these cellular components will be isolated from cardiac tissue and the Ca ions interaction with these components under various experimental conditions will be determined in the presence and absence of CG. Various experimental conditions to be employed are: alteration of ionic components of incubation media to induce membrane depolarization of sarcoplasmic reticulum or mitochondria, passage of electrical stimulation through incubation media containing sarcoplasmic reticulum or mitochondria, and the alteration of Ca ions concentration of media containing the modulatory proteins.